Dark Matter Gets Darker: New Measurements Confound Scientists

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New measurements of tiny galaxies contradict scientists' best
model of dark matter, further complicating the already mysterious
picture of the stuff that is thought to make up 98 percent of all
matter in the universe.

Dark matter, the invisible material thought to permeate the
universe, can only be indirectly detected through its
gravitational pull on the normal matter that makes up stars and
planets.

Despite not knowing exactly
what dark matter is, scientists have gradually built up a
good model to describe its behavior. The model envisions dark
matter made up of cold, slow-moving exotic particles that clump
together because of gravity.

This " cold
dark matter " model has done remarkably well describing how
dark matter behaves in most situations. However, it breaks down
when applied to mini " dwarf
galaxies," where dark matter appears more spread out than it
should be, according to the theory.

In a new study, researchers calculated the mass distribution of
two dwarf galaxies using a new method that did not rely on any
dark matter theories. The scientists studied the Fornax and
Sculptor galaxies, which orbit the Milky Way.

According to the model, the centers of galaxies should be packed
with dense clumps of the invisible matter. But dark matter
appears to be spread evenly throughout Fornax and Sculptor, as
well as other dwarf galaxies whose mass distributions have been
measured in other ways.

"If a dwarf galaxy were a peach, the standard cosmological model
says we should find a dark matter 'pit' at the center,"
researcher Jorge Peñarrubia of England's University of Cambridge
said in a statement. "Instead, the first two dwarf galaxies we
studied are like pitless peaches."

The measurements suggest that some part of the theoretical model
may have to be revised.

"Our measurements contradict a basic prediction about the
structure of cold
dark matter in dwarf galaxies," said study leader Matt Walker
of the Harvard-Smithsonian Center for Astrophysics in Cambridge,
Mass. "Unless or until theorists can modify that prediction, cold
dark matter is inconsistent with our observational data."

Dwarf galaxies like Fornax and Sculptor are especially good
places to study dark matter, because they are thought to be
almost entirely made up of the stuff. Only one percent of matter
in a dwarf galaxy is thought to be the normal matter that makes
up stars.

To determine where and how much dark matter inhabits the dwarf
galaxies, the researchers studied the motions of 1,500 to 2,500
visible stars, which reflect the gravitational forces acting on
them from dark matter.

Some researchers have suggested that when dark matter interacts
with normal matter it may tend to spread out, thus decreasing the
density of dark matter in the
centers of galaxies. However, so far, the cold dark matter
model doesn't predict this.

Either normal matter affects dark matter more than scientists
thought, or it isn't cold and slow-moving, the researchers said.

"After completing this study, we know less about dark matter than
we did before," Walker said.

The findings will be published in an upcoming issue of The
Astrophysical Journal.

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